1. Field Of The Invention
This invention relates to a magnetoresistive head assembly. More particularly, the invention relates to apparatus and a method for suppressing head-assembly playback signal error induced by an asperity in a magnetic recording medium.
2. Description Relative to the Prior Art
Magnetoresistive (MR) reproduce heads have gained wide acceptance in the magnetic recording field since they were disclosed in U.S. Pat. No. 3,493,694, issued to Hunt in 1970. An MR head is characterized by high output and low noise, making it particularly attractive for reproducing short wavelength signals. It may be fabricated by thin film deposition techniques allowing the relatively inexpensive manufacture of multitrack heads with narrow track widths for high density applications.
As taught in the patent to Hunt, the resistivity, .rho., of an elongated MR element can be expressed as: EQU .rho.=.rho..sub.o +.DELTA..rho. cos.sup.2 .theta., where
.rho..sub.o is the isotropic resistivity of the bulk material, PA0 .DELTA..rho. is the magnetoresistive coefficient (approximately 0.02 for Permalloy), and PA0 .theta. is the angle between the magnetization, M, of the MR element and measuring current, I, flowing through the element.
Since the magnetoresistive effect obeys a square law, it is desirable to establish an equilibrium angle between M and I of approximately 45.degree. to obtain essentially a linear mode of operation. Commonly, an anisotropy is established whose easy axis coincides with the longitudinal axis of the MR element and an external magnetic bias field is used that is transverse to the easy axis. The magnetic field source can be, for example, a thin film magnetic head or, as is also taught by Hunt, a permanent magnet, an electromagnet, a solenoid, etc.
Thermal fluctuations can produce a resistance change in an MR element. A thermally induced resistance change is an effect which, if it falls in the proper frequency range, can be interpreted erroneously as a signal produced by magnetic flux.
It is known in the art to compensate for ambient temperature changes by employing two MR elements interconnected to a differential amplifier to provide so-called common mode rejection of thermal effects. With both MR elements susceptible to the same ambient conditions, any thermally induced ambient effect in an MR element arranged for sensing magnetic flux transitions can be removed by subtracting from its output the output of a second MR element arranged to respond to temperature change but not to flux transitions. For more on this, see U.S. Pat. Nos. 3,860,965 and 3,979,775.
Although common-mode rejection techniques can compensate for low-frequency ambient temperature changes in an operating environment, a magnetic recording medium itself can be a major source of thermally induced resistance change in an MR element. With a magnetic tape or disk, for example, any slight imperfection or localized asperity in the oxide coating on the recording medium can cause "hot spots". Dust, lint, and other minute foreign matter on a magnetic recording surface may also cause hot spots. Prior art techniques intended to compensate for ambient temperature changes in the operating environment are not capable of compensating for localized media-induced effects. This is primarily for the reason that common-mode-rejection operates on the basis that two or more MR elements experience the same thermal effects simultaneously whereas localized asperities in a recording medium are normally so small that they would not be sensed by more than one MR element at any one time.
U.S. Pat. No. 4,040,113, on the other hand, discloses a magnetoresistive head assembly that is intended to compensate for temperature fluctuations due to variations in the roughness of the magnetic recording medium. For that purpose, the head assembly has a center-tapped electrical connection for dividing the MR element into two equal portions. As with other arrangements operated in a common rejection mode, however, U.S. Pat. No. 4,040,113 teaches that thermally induced fluctuations must be the same in each half of the head assembly in order to suppress adverse media effects. In other words, the common mode rejection of the '113 patent would not be effective in suppressing the effects of localized media asperities, i.e., asperities of small size. Notwithstanding this disadvantage, the magnetoresistive head assembly of U.S. Pat. No. 4,040,113 suffers from a further disadvantage in that the equilibrium angle between the easy axis of magnetization and the direction of current flow is achieved by providing a "barber-pole" pattern of highly conductive equipotential strips on the MR element. This pattern, which forces current to flow at a desired equilibrium angle with respect to the easy axis of the MR element, prevents the MR element from sensing magnetic flux wherever the conductive strips are located. In other words, the sensitivity of the head assembly is diminished in proportion to the surface area of the MR element that is covered by the "barber-pole" pattern of current-forcing strips.